Create a grid layout for IGList

[zhubofei]

Rebase from #140:

Create a flow layout for IGList that does not newline sections. Closes #3

• Minimum inter-item spacing
• Minimum line spacing
• Constant item size with constant layout time
• Update layout on insert/delete/move
• Unit Test

[zhubofei]

Still no coverall 😂

[jessesquires]

Thanks @zhubofei ! 💯

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[jessesquires]

@rnystrom -- Are we targeting this for the 2.0 release?

[rnystrom]

@jessesquires ya I think so, sound good? Importing today

[jessesquires]

@rnystrom - don't forget to update the CHANGELOG 😄.

Maybe we could merge #234 first though? 😊

[rnystrom]

Updated changelog, fixed some compiler errors from our strict build settings, and created a local var for self.collectionView in the attributes-for-rect method. Landing now.